1. Field of the Invention
This invention relates to a zoom lens, and particularly to a so-called four-unit zoom lens which appropriately uses a thin layer of resin in some lens surfaces in the lens system to thereby correct various aberrations well and which has a great aperture having F number of about 1.7 at the wide angle end and moreover has a wide angle of view (angle of view at the wide-angle end 2.omega.=58.degree. to 70.degree.) and has good optical performance over the entire variable power range of a variable power ratio (zooming ratio) as high as about 12 to 35 and which is thus suitable for a TV camera, a photographic camera, a video camera or the like.
2. Related Background Art
Zoom lenses having a great aperture, high variable power and moreover high optical performance have heretofore been required for TV cameras, photographic cameras, video cameras, etc.
In addition to this, particularly in color TV cameras for broadcasting, importance has been attached to operability and mobility, and in response to such requirement, a compact CCD (solid state image pickup device) of 2/3 inch or 1/2 inch has become the mainstream as an image pickup device.
In this CCD, the entire image pickup range has substantially uniform resolution and therefore, for a zoom lens using it, it is required that the various aberrations be well corrected from the center of the image field to the periphery of the image field and the resolution be substantially uniform.
For example, in a TV zoom lens, it is desired that of the various aberrations, particularly astigmatism, distortion and chromatic aberration of magnification be corrected well and the entire image field have high optical performance. It is further desired that the TV zoom lens have a great aperture, a wide angle of view and a high variable power ratio and moreover the entire lens system be compact and light in weight and have a long back focal length in order that a color resolving optical system and various filters may be disposed forwardly of image pickup means.
Of zoom lenses, a so-called four-unit zoom lens comprising, in succession from the object side, a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for zooming, a third lens unit of positive or negative refractive power for correcting the image plane fluctuating with zooming, and a fourth lens unit of positive refractive power for imaging is relatively easy to give a high variable power ratio and a great aperture and therefore is often used as a zoom lens for a color TV camera for broadcasting.
Of four-unit zoom lenses, a four-unit zoom lens having F number of the order of 1.6 to 1.9, a great aperture ratio and high variable power of a variable power ratio of the order of 13 is proposed, for example, in Japanese Laid-Open Patent Application No. 54-127322.
In a zoom lens, to obtain a great aperture ratio (F number 1.7 to 1.8) and a high variable power ratio (variable power ratio 12 to 35) and a wide angle of view (angle of view at the wide angle end: 2.omega.=58.degree. to 70.degree.) and moreover high optical performance over the entire variable power range, it is necessary to appropriately set the refractive power and lens construction of each lens unit.
Generally, to obtain high optical performance in which aberration fluctuation is small over the entire variable power range, it becomes necessary to increase, for example, the number of lenses in each lens unit to thereby increase the degree of freedom in the design for aberration correction.
Therefore, if an attempt is made to achieve a zoom lens having a great aperture ratio and a wide angle as well as a high variable power ratio, the number of lenses is unavoidably increased and there arises the problem that the entire lens system becomes bulky and thus, it becomes impossible to meet the desire for compactness and light weight.
Regarding the wider angle of a zoom lens, the correction of chromatic aberration of magnification among the various aberrations affecting the imaging performance becomes the greatest point at issue. This is because chromatic aberration of magnification affects by the tangent of the angle of view in the area of a primary chromatic aberration coefficient.
FIGS. 14 to 17 of the accompanying drawings show the optical paths of a conventional four-unit zoom lens (focal length=8.5 mm at the wide angle end (maximum wide-angle state), variable power ratio=15 times) at the wide-angle end (maximum wide-angle state) to the telephoto end (maximum telephoto state) thereof. FIG. 14 shows the wide-angle end (focal length fw), FIG. 15 shows the medium focal length fm (=fw.times.Z.sup.1/4) when the zoom ratio is Z, FIG. 16 shows the F drop focal length fd for which F number becomes reduced, and FIG. 17 shows the telephoto end.
Also, FIG. 13 of the accompanying drawings shows changes in the chromatic aberration of magnification of the same zoom lens resulting from a focal length change. In FIG. 13, there are shown four zoom positions, i.e., the wide-angle end (fw), the medium fm (fw.times.Z.sup.1/4) the F drop focal length fd and the telephoto end ft. As shown in FIG. 13, chromatic aberration of magnification is great plus as the entire image field at the wide-angle end (focal length fw). From the wide-angle end fw toward the medium fm, the F drop focal length fd and the telephoto end (focal length ft), it sequentially changes in the direction of minus. At this time, chromatic aberration of magnification passes the 0 zoom position, and at the telephoto end, the value of minus becomes greatest as the entire image field. Also, the chromatic aberration of magnification at the wide-angle side has a curve-like change for the angle of view, and g-line changes toward the minus side in the vicinity of the maximum angle of view for e-line (reference wavelength).
Accordingly, if at the wide-angle side, chromatic aberration of magnification is balanced at a certain medium angle of view, the amount of separation of g-line will become great and become a blue flare component inside it, and the amount of separation of c-line will become great and become a red flare component outside it. This tendency becomes more remarkable when the downsizing, a wider angle and a higher magnification are aimed at, and it becomes difficult to obtain a substantially uniform image quality on the entire image field.
Description will now be made of the mechanism by which the above-described high-order chromatic aberration of magnification at the wide-angle side occurs.
As shown in FIG. 17, in a first lens unit F, an on-axis marginal ray R1 passes the highest position at the telephoto end and therefore, it is necessary to sufficiently suppress the creation of spherical aberration and on-axis chromatic aberration in the first lens unit F. Accordingly, in a high magnification zoom lens having an angle of view 2.omega.=58.degree. to 70.degree. at the wide-angle end and a variable power ratio of the order of 12 to 35, the first lens unit F generally adopts a construction in which a negative lens formed of a material having a small Abbe's number is disposed at the object side and a plurality of positive lenses including a material having a great Abbe's number and a great abnormal dispersing property are disposed at the image side.
As shown in FIGS. 14 and 15, at the wide-angle side, an off-axis light beam passes a high position in the first lens unit F, is jumped down by a negative lens G1, and is jumped up by positive lenses G2 to G5. The negative lens G1 is small in the Abbe's number of its material and therefore, the jump-down at the short wavelength side thereof suddenly increases in the vicinity of the most peripheral angle of view, whereas the positive lenses G2 to G5 are great in the Abbe's number of their materials and great in the abnormal dispersing property thereof and therefore, the jump-up at the short wavelength side is deficient. As the result, as shown in FIG. 13, at the wide-angle side, g-line appears in the vicinity of the most peripheral angle of view as high-order chromatic aberration of magnification suddenly changing to minus. If for downsizing, the refractive power of the first lens unit F is strengthened as a reduction system, this tendency will become more remarkable because the refractive power of the aforementioned negative lens and of the aforementioned positive lenses becomes stronger.
As means for mitigating such high-order chromatic aberration of magnification, it may be mentioned to add, in FIG. 14, a negative lens formed of a material great in Abbe's number to the vicinity of a positive lens G2 in which the incidence height h2 of an off-axial light beam is great to make it share achromatism, and suppress the creation of a high-order component. Also, if a negative lens formed of a material great in Abbe's number is added to the vicinity of positive lenses G2 to G5 having a focal length fm at the variable power ratio Z.sup.1/4 of FIG. 5 and in which the incidence height h2 of the off-axial light beam suddenly increases, the jump-down by the aforementioned negative lens increases from the wide-angle end to the focal length fm at the variable power ratio Z.sup.1/4 and therefore, in addition to the mitigation of the high-order component by the sharing of achromatism, the fluctuation of chromatic aberration of magnification from the wide-angle end to the focal length fm at the variable power ratio Z.sup.1/4 toward the minus side and the fluctuation of distortion toward the plus side can be mitigated. Further, if the aforementioned negative lens added is formed into a meniscus shape concave toward the object side, high-order chromatic aberration of magnification can be corrected more effectively owing to the jump-up effect of the off-axial light beam by the image side convex surface of the aforementioned negative lens.
However, there has been the problem that the addition of the negative lens increases the full length of the first lens unit F and accordingly, increases the effective diameter of the first lens unit F, and this leads to the bulkiness of the zoom lens.